Method for patterning submicron pillars

ABSTRACT

The present invention provides for a method to pattern and etch very small dimension pillars, for example in a memory array. When dimensions of pillars become very small, the photoresist pillars used to pattern them may not have sufficient mechanical strength to survive the photoresist exposure and development process. Using methods according to the present invention, these photoresist pillars are printed and developed larger than their intended final dimension, such that they have increased mechanical strength, then are shrunk to the desired dimension during a preliminary etch performed before the etch of underlying material begins.

BACKGROUND OF THE INVENTION

The invention relates to a method to pattern and etch submicron pillarsusing photolithography and etch techniques.

In semiconductor devices it may be necessary to pattern and etchsubmicron pillars. An example is found in Herner et al., U.S. patentapplication Ser. No. 10/326,470, hereinafter the '470 application (sinceabandoned), herein incorporated by reference, which requires an array ofpillars having submicron dimensions to be formed using conventionalphotolithography and etch processes. Photoresist is patterned anddeveloped into photoresist features, which are then etched into anunderlying material. Even at very small critical dimensions (CDs),photoresist lines are mechanically strong enough to survive theexposure, development, and the cleaning and drying process that followsbefore the etch is performed. At the same CDs, however, photoresistpillars have less mechanical strength and may topple before the etch canbe performed.

There is a need, therefore, for a method to pattern and etch very smallpillars using standard photolithography and etch methods.

SUMMARY OF THE PREFERRED EMBODIMENTS

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Ingeneral, the invention is directed to a method to pattern and etch verysmall dimension pillars.

A first aspect of the invention provides for A method for forming areduced CD pillar, the method comprising forming a layer of etchablematerial; forming a layer of photoresist over the etchable material;patterning and developing the photoresist layer to form a photoresistpillar, the photoresist pillar having a first width no larger than about0.3 micron; shrinking the photoresist pillar to a second width smallerthan the first width; and etching the etchable material to form anetched pillar.

Another aspect of the invention provides for a method for etchingreduced CD pillars, the method comprising forming a layer of etchablematerial; forming a layer of photoresist over the etchable material;patterning and developing the photoresist layer to form a plurality ofphotoresist pillars, each photoresist pillar having a largest surfacedimension equal to a first width, the first width no larger than about0.3 micron; before etching the etchable material, shrinking thephotoresist pillars until the largest surface dimension is a secondwidth less than the first width; and etching the etchable material toform etched pillars.

Yet another aspect of the invention provides for a method for forming amemory array, the method comprising forming a layer of etchablematerial; forming a layer of photoresist over the etchable material;patterning and developing the photoresist to form a plurality ofphotoresist features, each photoresist feature having a largestpatterned dimension about equal to a first width, the first width lessthan about 0.3 micron; shrinking the photoresist features to a shrunkwidth, the shrunk width smaller than the first width; etching etchedfeatures in the etchable material; and forming the memory arraycomprising a plurality of memory cells, wherein each memory cellcomprises one of the etched features.

An embodiment of the invention provides for a method for forming amonolithic three dimensional memory array, the method comprising a)forming a first memory level by a method comprising i) forming a firstetchable layer; ii) forming a photoresist layer over the first etchablelayer; iii) patterning and developing the photoresist layer to formphotoresist pillars, each pillar having about a first width; iv)shrinking the photoresist pillars until each pillar has about a secondwidth, the second width less than the first width; and v) etching thefirst etchable layer to form etched pillars; and b) monolithicallyforming a second memory level above the first memory level.

A related embodiment of the invention provides for a monolithic threedimensional memory array comprising a) a first memory level formed overa substrate by a method comprising: i) forming a layer ofpolycrystalline or amorphous silicon; ii) depositing photoresist abovethe polycrystalline or amorphous silicon layer; iii) patterning anddeveloping the photoresist to form a plurality of photoresist pillarsarranged in an evenly spaced grid pattern, the largest patterneddimension of each pillar having about a first width; iv) beforesubstantially etching the layer of polycrystalline or amorphous silicon,reducing the first width to a second width less than the first width;and v) etching the layer of polycrystalline or amorphous silicon to formetched silicon pillars; and b) a second memory level monolithicallyformed on the first level.

Each of the aspects and embodiments of the invention described hereincan be used alone or in combination with one another.

The preferred aspects and embodiments will now be described withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a prior art memory cellaccording to the '470 application.

FIGS. 2 a and 2 b are perspective views illustrating formation of aplurality of patterned pillars formed above and aligned with a pluralityof patterned rails.

FIGS. 3 a and 3 b are perspective views showing a photoresist line andpillar. FIGS. 3 c and 3 d show the same photoresist pillar after CDshrink and after etch of the underlying material.

FIGS. 4 a through 4 c are perspective views illustrating CD shrink ofpillars according to a preferred embodiment of the present invention.

FIGS. 5 a through 5 f are cross-sectional views illustrating steps information of a memory level in a monolithic three dimensional memoryarray in which a plurality of pillars is patterned and etched accordingto embodiments of the present invention.

FIGS. 6 a and 6 b are cross-sectional views illustrating polarities ofdiodes that may be used in a monolithic three dimensional memory array.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The well-known techniques of photolithography and etch are used topattern features in semiconductor devices. This process begins with aphotomask blank, typically made from quartz and coated with an opaquematerial like chrome. To make a photomask, the chrome is etched, leavingbehind the desired pattern in chrome.

On a substrate, typically a wafer, a layer of the material that is to beetched into the desired pattern is deposited or grown on a surface,typically over a monocrystalline silicon wafer. This layer of material,in which the patterned features are ultimately intended to be formed,will be referred to in this discussion as the target layer. A layer ofphotoresist, a photoactive material, is deposited above the targetlayer. During exposure, the photomask transmits light where the chromehas been etched away and blocks it where the chrome remains, so someareas of photoresist are exposed to light, while others are not. Scalingtakes place during this exposure or printing process: A feature in thephotomask is typically about four or five times the size of thecorresponding printed feature.

Photoresist is chemically changed by exposure to light. Exposure isfollowed by a development process, in which the exposed photoresist ischemically removed, while the unexposed photoresist remains. At thispoint the photomask pattern that was etched in chrome has beentransferred to photoresist features.

The next step is an etch step, in which the surface is exposed to anetchant that will etch the target layer. Target layer material coveredby photoresist features is protected from the etchant, while the exposedareas are etched away. In this way the pattern of the photoresistfeatures is finally transferred to the target layer.

A variety of factors affect the smallest patterned width or CD of thephotoresist features that can be formed by exposure and development. Thetime and light energy used during exposure is called the dose, and ahigher dose will produce smaller CDs. The development process can beallowed to proceed for a shorter or longer time, where more developingtime results in smaller CDs.

It is also possible, after the development process is complete, to use apreliminary etch, before the actual etch of the target material starts,to reduce the CD of photoresist features, typically used to reduce thewidth of patterned lines. Such CD reduction, which takes place afterprinting and developing of the photoresist but before etch of the targetlayer, will be referred to as CD shrink.

The CD of patterned features, for example the width of patterned lines,is generally limited by the wavelength of the light used duringprinting. Features smaller than a half-wavelength cannot be successfullyresolved in the printing step. The methods of altering dose, developmenttime, and CD shrink have been used to create photoresist lines with awidth narrower than the width that can be successfully printed.

CD shrink has traditionally been used to reduce line widths in logicdevices. Shrinking gate length, for example, makes for faster turn-ontimes for devices.

The '470 application describes a monolithic three dimensional memoryarray having multiple stacked memory levels formed above a substrate.Each memory level comprises densely packed memory cells like those shownin FIG. 1. A vertically oriented diode 30 and antifuse 16 in series aredisposed between a bottom conductor 20 and a top conductor 40. Top andbottom conductors 20 and 40 preferably comprise adhesion layers 4 and18, respectively, preferably of titanium nitride, and conductive layers6 and 22, respectively, preferably of tungsten. The diode is formed of apolycrystalline semiconductor material such as silicon, and is in theform of a pillar. Variations on this memory cell are described in Herneret al., U.S. application Ser. No. 10/955,549, “Nonvolatile Memory CellWithout a Dielectric Antifuse Having High- and Low-Impedance States,”filed Sep. 29, 2003 and hereinafter the '549 application; Herner et al.,U.S. application Ser. No. 10/954,577, “Junction Diode Comprising VaryingSemiconductor Compositions,” filed Sep. 29, 2004 and hereinafter the'577 application; and Herner et al., U.S. application Ser. No.11/015,824, “Nonvolatile Memory Cell Comprising a Reduced HeightVertical Diode,” filed Dec. 17, 2004, and hereinafter the '824application, all hereby incorporated by reference.

Turning to FIG. 2 a, to form a memory level of the cells described inany of these applications, a plurality of first conductors is formed,for example by depositing a conductive stack, then patterning andetching that stack to form a plurality of substantially parallel,substantially coplanar bottom conductors 32. Tungsten is an advantageousmaterial for use in bottom conductors 32. A dielectric material 34 isdeposited over and between bottom conductors 32, filling the gapsbetween them. Planarization is performed, for example by chemicalmechanical planarization (CMP), to remove overfill of dielectricmaterial, and to coexpose dielectric material 34 and bottom conductors32.

Next a stack of semiconductor material, for example silicon, isdeposited, then patterned and etched to form a plurality of pillars 36.(A barrier layer, for example of titanium nitride, may be formed betweeneach pillar 36 and the underlying bottom conductor 32. The barrier layeris not shown.) The resulting structure is shown in FIG. 2 b. Nextdielectric material (not shown) is deposited over and between thepillars 36, filling the gaps between them. A planarizing stop isperformed, for example by CMP, to remove overfill of dielectric materialand to expose the tops of pillars 36.

To complete the memory level top conductors (not shown) will be formedas bottom conductors 34 were formed, by depositing and patterningconductive material into substantially parallel, substantially coplanartop conductors, which are preferably substantially perpendicular tobottom conductors 34.

Patterning and etching pillars like the semiconductor pillars 36 shownin FIG. 2 b at submicron dimensions is unconventional and introduces newchallenges. FIG. 3 a shows a patterned photoresist line 40 with havingwidth W. FIG. 3 b shows a photoresist pillar 42 having the same width W.Both the photoresist line 40 and the photoresist pillar 42 are shownafter exposure and development. The long dimension of photoresist line40 gives it more mechanical strength than photoresist pillar 42. To stopthe developing process, the wafer is conventionally sprayed withdeionized water, then spun dry. Photoresist pillar 42 is subjected tomechanical stresses during this wash and spin that make it susceptibleto mechanical failure, and photoresist pillar 42 may topple or slump.Photoresist line 40 is stronger and less vulnerable to such failure.

The photoresist pillar 42 could be strengthened by making it shorter,reducing its aspect ratio. Reducing the thickness of photoresist has thedisadvantage, however, that less photoresist remains to protect theunderlying material when it is to be etched; some amount of photoresistis normally consumed during this etch.

Patterning submicron pillars thus introduces the new problem thatphotoresist pillars may not have mechanical strength sufficient tosurvive the process of photoresist exposure and development. Thetechniques of CD shrink, performing a preliminary etch to reduce thewidth of a photoresist pillar after exposure and development arecomplete, can be used to address this new problem.

In aspects of the present invention, pillars are exposed and developedwith CDs larger than those eventually desired in the patterned pillars.The larger photoresist pillars formed after exposure and development aremechanically stronger and less prone to slumping. Additionally, betterpattern fidelity is achieved when printing with larger CDs.

Once the developing process is finished, the etch process can begin. Apreliminary etch step, which etches the photoresist pillars 42 as inFIG. 3 c, is performed. This preliminary etch should have an isotropiccomponent. The narrower pillars are formed in a low pressure environmentand are not subject to mechanical stress during this etch, and thus arenot prone to slumping despite their high aspect ratio. Once the CD ofthe photoresist pillar has been sufficiently reduced, a second etch isperformed to etch target layer 44 at the reduced CD, as shown in FIG. 3d. Had the etch of target layer 44 been performed using the originallarger size of the photoresist pillars, in an equally spaced array ofsuch pillars, gaps between the pillars would have higher aspect ratios,increasing the difficulty of filling those gaps, and the pillarspatterned in target layer 44 would be prone to bridging defects.

To summarize, what has been described is a method for forming a reducedCD pillar, the method comprising forming a layer of etchable material;forming a layer of photoresist over the etchable material; patterningand developing the photoresist layer to form a photoresist pillar, thephotoresist pillar having a first width no larger than about 0.3 micron;shrinking the photoresist pillar to a second width smaller than thefirst width; and etching the etchable material to form an etched pillar.

The pillars shown in the figures of this discussion in general appear tobe substantially cylindrical. Rectangular features formed with featuresize in both dimensions less than about 0.25 micron using standardphotomasking techniques tend to be substantially cylindrical, regardlessof the shape of the corresponding feature in the photomask. Below thisdimension, current photolithography techniques tend to round any sharpcorners on features. It is believed that this rounding occurs becausephotons used in the process diffract around the features in thephotomask.

A pillar, as the word is used in this discussion, is distinct from aline in that no width of the cross-section (parallel to the wafersurface) in any direction is more than three times the width of thecross-section in any other direction. A patterned pillar is alsorelatively small, such that no surface dimension of a pillar is greaterthan about 0.3 micron. As will be seen, in most preferred embodiments,no surface dimension of a pillar is greater than about 0.25 micron.

In one particularly advantageous embodiment, shown in FIG. 4 a, targetlayer 44 is deposited first, followed by a layer of bottomantireflective coating (BARC) 46, then by a layer of photoresist 42.Photoresist 42 is exposed and developed into oversize pillars 42, shownin FIG. 4 b. BARC is conventionally used to prevent reflection duringprinting. BARC layer 46, of course, must be etched before target layer44 can be etched. The etch chemistries of BARC and of photoresist arevery similar, so it is possible to etch BARC layer 46 and shrinkphotoresist pillars 42 in the same etch step, with the result shown inFIG. 4 c. Arrows indicate that this etch with an isotropic component hascaused shrinkage of the photoresist pillars 42 in both height and width.This etch should be selective, with a high etch rate for photoresist andBARC but a very low etch rate for the material of target layer 44.

BARC is conventionally used to prevent unwanted exposure fromreflection. Use of BARC in embodiments of the present invention providesthe extra advantage that in the following etch, the additional thicknessof the BARC helps compensate for photoresist thickness lost during theCD shrink etch.

A detailed example will be provided describing fabrication of a memorylevel of a monolithic three dimensional array like the array of the '470application, the '549 application, the '577 application, or the '824application. Fabrication of this memory level will include pattern andetch of polysilicon pillars according to embodiments of the presentinvention. For completeness, specific details regarding materials,steps, and conditions will be provided. It will be understood by thoseskilled in the art, however, that many of these details can be modified,omitted, or augmented while the result still falls within the scope ofthe invention.

EXAMPLE

Fabrication of a single memory level is described. Additional memorylevels can be stacked, each monolithically formed above the one belowit.

Turning to FIG. 5 a, formation of the memory begins with a substrate100. This substrate 100 can be any semiconducting substrate as known inthe art, such as monocrystalline silicon, IV-IV alloys likesilicon-germanium or silicon-germanium-carbon, III-V alloys, II-VIIalloys, epitaxial layers over such substrates, or any othersemiconducting material. The substrate may include integrated circuitsfabricated therein.

An insulating layer 102 is formed over substrate 100. The insulatinglayer 102 can be silicon oxide, silicon nitride, high-dielectric film,Si—C—O—H film, or any other suitable insulating material.

The first conductors 200 are formed over the substrate and insulator. Anadhesion layer 104 may be included between the insulating layer 102 andthe conducting layer 106 to help the conducting layer 106 adhere.Preferred materials for the adhesion layer 104 are tantalum nitride,tungsten nitride, titanium tungsten, sputtered tungsten, titaniumnitride, or combinations of these materials. If the overlying conductinglayer 106 is tungsten, titanium nitride is preferred as an adhesionlayer.

The next layer to be deposited is conducting layer 106. Conducting layer106 can comprise any conducting material known in the art, includingtantalum, titanium, tungsten, copper, cobalt, or alloys thereof.Titanium nitride may be used.

Once all the layers that will form the conductor rails have beendeposited, the layers will be patterned and etched using any suitablemasking and etching process to form substantially parallel,substantially coplanar conductors 200, shown in FIG. 5 a incross-section. In one embodiment, photoresist is deposited, patterned byphotolithography and the layers etched, and then the photoresist removedusing standard process techniques.

Next a dielectric material 108 is deposited over and between conductorrails 200. Dielectric material 108 can be any known electricallyinsulating material, such as silicon oxide, silicon nitride, or siliconoxynitride. In a preferred embodiment, silicon oxide is used asdielectric material 108.

Finally, excess dielectric material 108 on top of conductor rails 200 isremoved, exposing the tops of conductor rails 200 separated bydielectric material 108, and leaving a substantially planar surface 109.The resulting structure is shown in FIG. 5 a. This removal of dielectricoverfill to form planar surface 109 can be performed by any processknown in the art, such as chemical mechanical planarization (CMP) oretchback. At this stage, a plurality of substantially parallel firstconductors have been formed at a first height above substrate 100.

Next, turning to FIG. 5 b, vertical semiconductor pillars will be formedabove completed conductor rails 200. (To save space substrate 100 isomitted in FIG. 5 b; its presence will be assumed.) If conductivematerial 106 was tungsten, it is preferred to deposit barrier layer 110,preferably of titanium nitride, after planarization of the conductorrails. This layer can be formed in any conventional manner. Itsthickness can be, for example, about 20 to about 500 angstroms. Thethickness of barrier layer 110 is preferably about 200 angstroms.

Next semiconductor material that will be patterned into pillars isdeposited. The semiconductor material can be silicon, silicon-germanium,silicon-germanium-carbon, germanium, or other suitable semiconductors oralloys. Silicon is commonly used in the industry, so, for simplicity,this description will refer to the semiconductor material as silicon,but it will be understood that other materials may be substituted.

In preferred embodiments, the semiconductor pillar is a junction diode,comprising a bottom heavily doped region of a first conductivity typeand a top heavily doped region of a second conductivity type. The middleregion, between the top and bottom regions, is an intrinsic or lightlydoped region of either the first or second conductivity type. The diodeof FIG. 6 a has a bottom region 112 of P+ silicon, intrinsic region 114,and N+ top region 116. The diode of FIG. 6 b is reversed, having abottom region 112 of N+ (heavily doped n-type) silicon, intrinsic region114, and P+ top region 116. The middle region is intrinsic, or notintentionally doped, though in some embodiments it may be lightly doped.An undoped region will never be perfectly electrically neutral, and willalways have defects or contaminants that cause it to behave as ifslightly n-doped or p-doped. Such a diode can be considered a p-i-ndiode.

To form, for example, the diode of FIG. 6 a, a layer of heavily dopedn-type silicon 112 must be formed, shown in FIG. 5 b. This layer can beformed by any deposition and doping method known in the art. The siliconcan be deposited and then doped, but is preferably doped in situ byflowing a donor gas providing dopant atoms during deposition of thesilicon. In a preferred embodiment, this layer can range from about 100to about 1500 angstroms, preferably 400 angstroms, and have a dopantconcentration of about 1×10¹⁹ to about 2×10²¹ atoms/cm³, and preferablyabout 8×10²⁰ atoms/cm³.

In preferred embodiments, layers 114 and 116 are deposited in a singledeposition step. These layers are deposited as intrinsic, undopedsilicon; in preferred embodiments, silicon is deposited withoutintentional doping, yet has defects which render it slightly n-type.Layers 114 and 116 together have a thickness of between about 1400 andabout 4000 angstroms, preferably about 3300 angstroms. Note this is thethickness as-deposited. Some portion of the top of this layer, forexample about 800 angstroms, will be consumed in a subsequent CMP oretchback step, and will thus be thinner in the finished device. Heavilydoped layer 116 will be differentiated from intrinsic layer 114 in alater ion implantation step, described below. As there is no distinctionbetween layers 114 and 116 at this point, only layer 114 is labeled inFIG. 5 b.

In preferred embodiments a material that will act as a hard mask duringthe polysilicon etch is deposited next, for example silicon dioxidelayer 118. This layer is preferably between about 200 to about 800angstroms thick, more preferably about 400 angstroms thick. A layer 120of BARC is deposited on silicon dioxide layer 118. The thickness of theBARC is as preferably between about 200 and about 1000 angstroms, mostpreferably about 600 angstroms. Finally photoresist layer 122 isdeposited on BARC layer 120. The thickness of photoresist layer 122 isbetween about 1500 and about 6000 angstroms, preferably about 4000angstroms.

Turning to FIG. 5 c, using conventional photolithography methods,photoresist layer 122 is exposed and developed, forming photoresistpillars 122. In preferred embodiments, photoresist pillars 122 arearranged in an evenly spaced grid pattern. The term grid pattern means apattern in which the pillars are equally spaced in two directions, thedirection preferably substantially perpendicular to each other. Spacingin such a pattern is preferably substantially equal in each direction,but need not be. Each photoresist pillar will be used to as a templateto form a polysilicon diode from polysilicon layers 112, 114, and 116(this layer still to be formed by implantation step), and each of thesevertically oriented diodes will be aligned on one of the lower conductorrails 200. Some misalignment can be tolerated.

Pitch is the distance between adjacent instances of the same feature ina repeating pattern—the distance from the center of one line to thecenter of the next in an array of evenly spaced lines, for example. Aplurality of pillars arranged in an evenly spaced grid pattern thus hastwo pitches, one in each dimension of the grid.

Thus the pitch of the diodes to be formed in the direction perpendicularto bottom conductor rails 200, and therefore of the photoresist pillars122, should be substantially the same as the pitch of the bottomconductor rails 200. The pitch of the photoresist pillars 122 in thedirection parallel to bottom conductor rails 200 should be substantiallythe same as the pitch of the top conductor rails, which will be formedin a later step. These top conductor rails preferably will besubstantially perpendicular to bottom conductor rails 200 and have thesame pitch, though other directions and other pitches may be usedinstead.

The pitch of photoresist pillars 122 is preferably between about 0.05and about 1.0 micron, more preferably between about 0.5 micron and about0.09 micron. In one preferred embodiment, the pitch is between about0.29 and about 0.35 micron, preferably about 0.32 micron. In anotherpreferred embodiment, the pitch is between about 0.25 and about 0.28micron, preferably about 0.26 micron. In another preferred embodiment,the pitch is between about 0.16 and about 0.20 micron, preferably about0.18 micron.

In one example, if the pitch is about 0.32 micron, for example, thecritical dimension of a photoresist pillar after exposure anddevelopment is between about 0.17 and about 0.20 micron, preferablybetween about 0.18 and about 0.19 micron, while the gap between pillars122 is between about 0.13 and about 0.14 micron. In another example, ifthe pitch is about 0.26 micron, the width of the photoresist pillar 122after exposure and development is about 0.15 micron, while the gapbetween pillars 122 is about 0.11 micron. Note that the criticaldimension and gap width may vary across an individual wafer. FIG. 5 cshows the structure after exposure and development of photoresist.

Next a CD shrink etch is performed. In preferred embodiments, the BARClayer 120 is etched and the CD of photoresist pillars 122 reduced duringthis etch step. An etch can have varying degrees of isotropy. Aperfectly anisotropic etch etches only in the vertical direction, withno horizontal etch, while a perfectly isotropic etch etches verticallyand horizontally at the same rate. Using most conventional dry etchants,generally the horizontal etch rate cannot be higher than the verticaletch rate.

It will be recalled that in a preferred embodiment the thickness of BARClayer 120 is about 600 angstroms. The degree of isotropy will becontrolled such that the CD of a photoresist pillar 122 is reduced bythe desired amount during this 600-angstroms BARC etch. For example, ifthe width of a 0.18 micron photoresist pillar is to be reduced to 0.16micron during the BARC etch, then 0.02 micron must be etchedhorizontally, 0.01 micron from each side of the pillar. Thus in thisexample the ratio of vertical etch rate (600 angstroms) to horizontaletch rate (0.01 micron, or 100 angstroms) is 6:1.

A combination of HBr and O₂ can be used to etch BARC and photoresist.Increasing the proportion of O₂ increases the isotropy of this etch. Inone example, during the BARC etch, HBr was flowed at about 20 sccm withpressure of about 10 mTorr. Top power was about 250 watts while bottompower was about 50 watts.

In a series of tests, it was found that flowing 5 sccm of O₂ with the 20sccm of HBr produced no appreciable CD reduction when the 600 angstromBARC etch was complete. A flow of 7 sccm of O₂ reduced the width of thepillar by about 15 nm, about 10 sccm of O₂ by caused about a 25 nmreduction, about 12 sccm of O₂ caused about 30 nm of reduction, about 16sccm caused about 40 nm, and a flow of about 20 sccm of O₂ reduced thewidth of the pillars by about 50 nm when the 600 angstrom BARC etch wascomplete. In the present example, then between 7 and 10 would producethe desired reduction of about 20 nm. Other variables, such as equipmentused, pressure, temperature, etc., will also affect isotropy. Theskilled practitioner will be accustomed to optimizing etch conditionsaccording to equipment, the pattern to be etched, and other usualvariables.

Other etchants can be used in place of HBr, for example chlorine andCF₄. Increasing the O₂ percentage with either of these etchants alsoallows for a controllable increase in isotropy.

In the example just given, the percentage of O₂ is varied to control thedegree of isotropy during an etch. Other variables can be used to do so,however, including percentage overetch, pressure, ratio of source powerto bias power, and temperature, or any combination of these variables.

If no BARC layer is used, and the CD shrink etch is to be performed onphotoresist pillars 122 only without etching any underlying layer, thenit is advantageous for the CD shrink etch to be as isotropic as possibleto minimize loss of photoresist thickness.

Before the CD shrink etch, the photoresist pillar 122 had a first width.After the CD shrink etch is complete, the photoresist pillar 122 has asecond with smaller than the first width. The second width is preferably95 percent or less of the first width. The second width may be 90percent or less of the first width, and may be reduced to as much as 80or even 70 percent of the first width. Even greater reductions arepossible if desired.

The second width is preferably at least about 0.01 micron less than thefirst width, and may be 0.02 micron less than the first width, or asmuch as 0.04 or 0.06 micron less than the first width. Even greaterreductions are possible if desired.

Specifically, if the first width, before the CD shrink etch, was about0.18 to about 0.19 micron, then the second width, after the CD shrinketch, may be about 0.16 to about 0.17 micron. Similarly, if the firstwidth was about 0.15 micron, the second width may be about 0.13 micron.If the first width was about 0.11 micron, the second width may be about0.09 micron.

At this point the photoresist pillars 122 have been narrowed and BARClayer 120 has been etched through, as shown in FIG. 5 d. (The drawingsare designed to illustrate concept only, and should not be presumed tocorrectly portray scale or aspect ratio.) Next silicon dioxide layer 118is etched. Finally a silicon etch etches polysilicon layers 114, and112. Photoresist pillar 122 may have been partly etched away by the timethis etch begins, but a thickness of BARC layer 120 beneath it andsilicon dioxide layer 118 remain. Polysilicon etchants can be used thatare highly selective to silicon dioxide, so silicon dioxide layer 118functions as a hard mask. Barrier layer 110 of titanium nitride shouldbe etched in this step as well.

To summarize, a method has been described for etching reduced CDpillars, the method comprising forming a layer of etchable material;forming a layer of photoresist over the etchable material; patterningand developing the photoresist layer to form a plurality of photoresistpillars, each photoresist pillar having a largest surface dimensionequal to a first width, the first width no larger than about 0.3 micron;before etching the etchable material, shrinking the photoresist pillarsuntil the largest surface dimension is a second width less than thefirst width; and etching the etchable material to form etched pillars.

The photolithography techniques described in Chen, U.S. application Ser.No. 10/728,436, “Photomask Features with Interior Nonprinting WindowUsing Alternating Phase Shifting,” filed Dec. 5, 2003; or Chen, U.S.application Ser. No. 10/815,312, Photomask Features with ChromelessNonprinting Phase Shifting Window,” filed Apr. 1, 2004, both owned bythe assignee of the present invention and hereby incorporated byreference, can advantageously be used to perform any photolithographystep used in formation of a memory array according to the presentinvention.

Once the etch is complete, a conventional ashing and clean processremoves the remaining photoresist and BARC. The etched pillars,including silicon dioxide layer 118, polysilicon layers 112 and 114, andtitanium nitride barrier layer 110, remain. The structure at this pointis shown in FIG. 5 e.

In this example, silicon dioxide layer 118 and BARC layer 120compensated for the thickness of photoresist lost during the CD shrinketch, providing protection for underlying layers during the polysiliconetch. Skilled practitioners may choose to use or omit these layers, orsubstitute other materials for the same purpose. Such choices areroutinely made by those skilled in the art.

Turning to FIG. 5 f, dielectric material 108 is deposited over andbetween the semiconductor pillars 300, filling the gaps between them.Dielectric material 108 can be any known electrically insulatingmaterial, such as silicon oxide, silicon nitride, or silicon oxynitride.In a preferred embodiment, silicon dioxide is used as the insulatingmaterial.

Next the dielectric material on top of the pillars 300 is removed,exposing the tops of pillars 300 separated by dielectric material 108,and leaving a substantially planar surface. This removal of dielectricoverfill can be performed by any process known in the art, such as CMPor etchback. An ion implantation step performed at this point createsheavily doped top regions 116, in this example using a p-type dopant toform a P+ region. After implantation, this layer will preferably have adopant concentration of about 2×10¹⁹ to about 4×10²¹ atoms/cm³,preferably about 8×10²⁰ atoms/cm³.

In some embodiments a dielectric antifuse layer 125, preferably ofsilicon dioxide, is formed on heavily doped regions 116. In preferredembodiments, as shown, a silicon dioxide layer 125 is grown by oxidizingsilicon at the tops of heavily doped regions 116 at about 600 to about850 degrees C. for about 20 seconds to about two minutes, formingbetween about 15 and about 50 angstroms of silicon dioxide. Preferably,silicon dioxide layer 125 is formed by exposing the wafer to about 800degrees for about one minute in an oxygen-containing ambient. Layer 125could be deposited instead. As described in the '549 application, thisdielectric antifuse layer need not always be present.

Overlying conductors can be formed in the same manner as the underlyingconductors. The overlying conductors will be formed at a height abovethe height of the first conductors, and extend in a different directionfrom them, preferably substantially perpendicular to them.

To summarize, what has been described is a method for forming a memoryarray, the method comprising: forming a layer of etchable material;forming a layer of photoresist over the etchable material; patterningand developing the photoresist to form a plurality of photoresistfeatures, each feature having a largest patterned dimension about equalto a first width, the first width less than about 0.3 micron; shrinkingthe features to a shrunk width, the shrunk width smaller than the firstwidth; etching etched features in the etchable material; and forming thememory array comprising a plurality of memory cells, wherein each memorycell comprises one of the etched features.

At this point a first memory level has been formed. Additional memorylevels can be monolithically formed above the first, as described in the'470, '549 applications and the other incorporated references, forming amonolithic three dimensional memory array. For example, a secondplurality of pillars can be formed above the upper conductors, and athird plurality of conductors can be formed above them. The upperconductors of one memory level can serve as the lower conductors of anoverlying memory level, or an interlevel dielectric can be formedbetween them.

What has been formed is a monolithic three dimensional memory arraycomprising: a) a first memory level formed over a substrate by a methodcomprising: i) forming a layer of polycrystalline or amorphous silicon;ii) depositing photoresist above the polycrystalline or amorphoussilicon layer; iii) patterning and developing the photoresist to form aplurality of photoresist pillars arranged in an evenly spaced gridpattern, the largest patterned dimension of each pillar having about afirst width; iv) before substantially etching the layer ofpolycrystalline or amorphous silicon, reducing the first width to asecond width less than the first width; and v) etching the layer ofpolycrystalline or amorphous silicon to form etched silicon pillars; andb) a second memory level monolithically formed on the first level.

Such a monolithic three dimensional memory array can be formed by amethod comprising: a) forming a first memory level by a methodcomprising: i) forming a first etchable layer; ii) forming a photoresistlayer over the first etchable layer; iii) patterning and developing thephotoresist layer to form photoresist pillars, each pillar having abouta first width; iv) shrinking the photoresist pillars until each pillarhas about a second width, the second width less than the first width;and v) etching the first etchable layer to form etched pillars; and b)monolithically forming a second memory level above the first memorylevel.

Monolithic three dimensional memory arrays are described in Johnson etal., U.S. Pat. No. 6,034,882, “Vertically stacked field programmablenonvolatile memory and method of fabrication”; Johnson, U.S. Pat. No.6,525,953, “Vertically stacked field programmable nonvolatile memory andmethod of fabrication”; Herner, U.S. application Ser. No. 10/095,962,“Silicide-Silicon Oxide-Semiconductor Antifuse Device and Method ofMaking,” filed Mar. 13, 2002; Petti et al., U.S. application Ser. No.10/728,230, “Semiconductor Device Including Junction Diode ContactingContact-Antifuse Unit Comprising Silicide,” filed Dec. 3, 2003; Herner,U.S. application Ser. No. 10/954,510, “Memory Cell Comprising aSemiconductor Junction Diode Crystallized Adjacent to a Silicide,” filedSep. 29, 2004; and Petti, U.S. application Ser. No. 10/955,387, “FuseMemory Cell Comprising a Diode, the Diode Serving as the Fuse Element,”filed Sep. 29, 2004, all assigned to the assignee of the presentinvention and hereby incorporated by reference. As appropriate, themethods of the present invention can be used in conjunction with thememories described in these patents and applications to form monolithicthree dimensional memory arrays according to the present invention.

A monolithic three dimensional memory array is one in which multiplememory levels are formed above a single substrate, such as a wafer, withno intervening substrates. The layers forming one memory level aredeposited or grown directly over the layers of an existing level orlevels. In contrast, stacked memories have been constructed by formingmemory levels on separate substrates and adhering the memory levels atopeach other, as in Leedy, U.S. Pat. No. 5,915,167, “Three dimensionalstructure memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree dimensional memory arrays.

A monolithic three dimensional memory array formed above a substratecomprises at least a first memory level formed at a first height abovethe substrate and a second memory level formed at a second heightdifferent from the first height. Three, four, eight, or more memorylevels can be formed above the substrate in such a multilevel array.

The present invention has been described herein in the context of amonolithic three dimensional memory array. It will be apparent to thoseskilled in the art, however, that the methods of the present inventioncan be employed advantageously whenever pillars having very smalldimensions (smaller than 0.3 micron in every surface dimension, andespecially smaller than 0.2 micron) are to be patterned and etched usingconventional photolithography and etch processes. The methods of thepresent invention can be employed in two dimensional arrays, non-memoryuses, and to pattern individual shapes, not formed in arrays.

Detailed methods of fabrication have been described herein, but anyother methods that form the same structures can be used while theresults fall within the scope of the invention.

The foregoing detailed description has described only a few of the manyforms that this invention can take. For this reason, this detaileddescription is intended by way of illustration, and not by way oflimitation. It is only the following claims, including all equivalents,which are intended to define the scope of this invention.

1. A method for forming a reduced CD pillar, the method comprising:forming a conductor rail above a substrate, the conductor rail having arail width; forming a layer of etchable material above the conductorrail; forming a layer of photoresist over the etchable material;patterning and developing the photoresist layer to form a photoresistpillar having no cross-section width in any direction which is more thanthree times the cross-section width in any other direction, thephotoresist pillar having a first width no larger than about 0.3 micronand having a first aspect ratio, wherein the photoresist pillar isadapted to survive mechanical stresses encountered during awash-and-spin process, and wherein the first width is larger than therail width; spraying the photoresist layer with deionized water to stopthe developing; spinning the substrate to remove the deionized water;shrinking the photoresist pillar to a second width smaller than thefirst width to form a shrunken photoresist pillar while controllingisotropy during the shrinking step by using an O₂ based etch andadjusting the O₂ percentage used to control isotropy, wherein theshrunken photoresist pillar has a second aspect ratio greater than thefirst aspect ratio, and wherein the second width is substantially equalto the rail width; and etching the etchable material to form an etchedpillar, wherein the etched pillar is substantially aligned with theconductor rail and has a pillar width substantially equal to the railwidth.
 2. The method of claim 1 wherein the second width is less thanabout 95 percent of the first width and the shrinking step comprisesetching the photoresist pillar with any etch and controlling isotropyduring the etch.
 3. The method of claim 2 wherein a degree of isotropyof the etching of the photoresist pillar is controlled to obtain aselected amount of reduction in the first width.
 4. The method of claim3 wherein shrinking step is performed using a combination of HBr and O₂as an etchant and wherein the isotropy is controlled by adjusting the O₂percentage used to etch the photoresist pillar.
 5. The method of claim 4wherein the second width is at least about 0.02 micron less than thefirst width.
 6. The method of claim 1 wherein the etched pillar issubstantially cylindrical.
 7. The method of claim 1 wherein the etchablematerial comprises silicon or germanium.
 8. The method of claim 1further comprising forming a layer of bottom antireflective coatingbetween the layer of etchable material and the layer of photoresist, andremoving any of the coating not covered by the photoresist pillar duringthe shrinking step.
 9. The method of claim 8 further comprisingcontrolling isotropy to remove the coating not covered by thephotoresist pillar while obtaining a selected amount of reduction in thefirst width of the photoresist pillar.
 10. The method of claim 1 whereinthe layer of etchable material is formed above a monocrystalline siliconsubstrate.
 11. A method for etching a plurality of etched reduced CDpillars, the method comprising: forming a plurality of substantiallyparallel conductor rails above a substrate, each of the plurality ofsubstantially parallel conductor rails having a rail width and beingseparated from an adjacent rail by a rail pitch; forming a layer ofetchable material above the plurality of conductor rails; forming alayer of photoresist over the etchable material; patterning anddeveloping the photoresist layer to form a plurality of photoresistpillars, each photoresist pillar having a largest surface dimensionequal to a first width, the first width no larger than about 0.3 micronand having a first aspect ratio, wherein each photoresist pillar isadapted to survive mechanical stresses encountered during awash-and-spin process, and wherein the first width is larger than therail width; spraying the photoresist layer with deionized water to stopthe developing; spinning the substrate to remove the deionized water;before etching the etchable material, shrinking the photoresist pillarsto form shrunken photoresist pillars while varying isotropy during theshrinking step using an O₂ based etch and adjusting the O₂ percentageused to control isotropy until the largest surface dimension is a secondwidth less than the first width, wherein each shrunken photoresistpillar has a second aspect ratio greater than the first aspect ratio,and wherein the second width is substantially equal to the rail width;and etching the etchable material to form the plurality of etchedreduced CD pillars in a grid pattern, wherein each etched pillar issubstantially aligned with one of the plurality of conductor rails, hasa pillar width substantially equal to the rail width, and has a firstpillar pitch in a direction perpendicular to the plurality of conductorrails substantially equal to the rail pitch.
 12. The method of claim 11wherein the shrinking step comprises etching the photoresist pillars andvarying etch parameters during the etch to adjust isotropy.
 13. Themethod of claim 12 wherein the isotropy of the etch is varied during theetch to obtain a selected amount of reduction in the first width. 14.The method of claim 13 wherein the shrinking step uses an HBr and O₂etch and wherein isotropy is controlled by adjusting the O₂ percentageused in the etch of the photoresist pillars.
 15. The method of claim 14wherein the second width is at least about 0.02 micron less than thefirst width.
 16. The method of claim 11 further comprising forming alayer of bottom antireflective coating between the layer of etchablematerial and the layer of photoresist, and etching the coating duringthe shrinking step.
 17. The method of claim 16 further comprisingadjusting isotropy to remove the coating not covered by the photoresistpillars while obtaining a selected amount of reduction in the firstwidth of the photoresist pillars.
 18. The method of claim 11 wherein theetched pillars are substantially cylindrical.
 19. The method of claim 11wherein the etched pillars are evenly spaced in the grid pattern, suchthat each pillar has a second pillar pitch in a direction parallel tothe plurality of conductor rails substantially equal to the rail pitch.20. The method of claim 19 wherein the etched pillars have first andsecond pitches of about 0.6 micron or less.
 21. The method of claim 20wherein the etched pillars have first and second pitches of about 0.36micron or less.